Not Applicable
Not Applicable
1. Field of the Invention
This invention relates to a printed circuit board (PCB) configuration, which improves the electromagnetic compatibility/electromagnetic interference (EMC/EMI) performance of electromechanical relay circuits.
2. Description of the Related Art
Relay devices frequently switch large currents into inductive loads. Contact bounce and circuit interruption result in high back-EMF voltages and large currents across the relay contact. The relay contact essentially forms a spark gap with quenching performance that varies with the movement of the relay armature. Consequently, a whole spectrum of high-energy noise is created that is well known to be disruptive to logic and microprocessor circuits.
The inductive load circuits controlled by relay contacts are normally located outside the enclosure of the controlling logic, and some distance away. Because of the proximity of the relays to logic and microprocessor circuits, inductive and capacitive coupling mechanisms may exist which will introduce radio-frequency noise onto the relay circuits. The wiring for the logic circuits form antennas that will radiate any noise energy that may be present onto the relay circuits.
Referring to FIGS. 1 and 2, in classical analysis of EMC problems, there is always a source 2, a victim 4, and a coupling mechanism 6. The mechanism of inductively coupled noise and the current methods used to minimize its impact is discussed hereinbelow. Analogies can be drawn using an air-core radio-frequency (RF) transformer as the coupling mechanism 6. Analysis shows that the strength of the inductive coupling depends upon the mutual inductance. A circuit loop on a PCB will behave like a single-turn transformer. The source device circuit loop 3 will couple to the circuit loop 5 of the victim. Various techniques are currently employed to deliberately reduce the coupling effect of the PCB transformer.
Referring to FIGS. 3 and 4, the disruptive effects of relay contact circuits on logic and microprocessor circuits have been controlled by segregation of relay contact circuits 8 and electronic logic circuits 10. Segregation simply separates the sensitive logic circuits 10 from the larger currents switched by the relay circuits 8 and the resultant magnetic flux 12. Wiring loop 13 acts like a coupling transformer for magnetic flux 12.
Referring to FIG. 5, at times segregation includes separate PCBs 14 and 16, and partitioning the system with a magnetic shield 18 made of a ferrous material between the relay circuits 8 and the logic circuits 10. The magnetic shield 18 provides a low-reluctance path for the magnetic flux 12, containing it largely within the shield 18 sub-enclosure. Segregation approaches have been successful, but they require a product to be large and bulky, and accessibility for product service can be compromised.
EMI filters have been used to reduce noise that is directly conducted by the signal wiring. Such devices are presently available off-the-shelf as line filters for power supply applications. These devices are often bulky because relay contact circuits are normally required to handle large currents. EMI filters can be connected to the wiring loop 13 shown in FIG. 3.
Referring to FIGS. 6 and 7, presently available metal oxide varister (MOV) devices can be used that reduce inductive kickback voltages resulting from interrupting the current to user-connected equipment. They also define the path of the inductive-discharge currents to limit the disruptive effects to nearby electronic circuits. MOV devices are compact and cost effective, but have a finite service life. However, by diverting the energy away from the relay contacts, the service life of the relay can be increased. FIG. 6 illustrates a circuit without an MOV device in which a large discharge current 20 passes through the relay contact 9. The noise generated in the spark gap (relay contact 9) couples inductively to nearby logic circuits. FIG. 7 illustrates a circuit using an MOV device 22 in which discharge current 21 is much smaller than discharge current 20 because the energy is diverted through the MOV device 22. Smaller discharge current 21 results in a smaller noise current and less potential disruptions of nearby logic circuits.
Referring to FIG. 8, placing the forward and return traces for the relay contacts on closely spaced parallel conductors 24 reduces the inductive area of the PCB circuit and thus reduces inductive coupling in comparison to wiring loop 13 shown in FIG. 4. A further enhancement of this technique places the two paths of the circuit on opposite sides of the PCB. This reduces the inductive area to the thickness of the PCB.
Referring to FIG. 9, the use of multilayer boards 25 has been found to greatly reduce the EMI generated by PCBs. Top wiring or etched layer 26 contains the signal wiring traces 28 and associated logic circuits 10. The ground and power planes 30 are separated by insulation, and a bottom wiring/etched layer 31 can be included. The ground and power planes 30 allow the return currents 32 to form directly adjacent to each signal line 28, with each return circuit 32 finding the path of least impedance that closely mirrors each signal trace 28. The signal traces 28 and these mirror currents 32 form the smallest inductive area and thus minimize the effects of inductive coupling and electromagnetic disturbances. The path of least impedance for rapid changing currents is the path that forms the smallest inductive area, directly under the signal trace. The mirror currents automatically form the paths that achieve minimum inductive coupling. If the signal path must cross a gap in the planes, the mirror currents are forced to form a larger area and generate much more inductive noise.
Referring to FIG. 10, when a toggling logic output drives a logic input, there is a finite return current 34. The return current 34 moves through the ground path 36. If a cable 38 is connected to the driven gate, even on the logic chip ground lead, it will become an antenna radiating RF energy. The ground path 36 forms an inductor with small but finite inductance. This distributed inductance forms an autotransformer. The finite currents changing in the finite inductance produce voltages 39 on the connected cable that may radiate several milliwatts of power.
The autotransformer coupling mechanism described above is generally termed common impedance coupling. The 3 or 5-volt logic transitions are not the problem. If the toggling output of a logic gate was directly connected to twisted-pair of unshielded cable it would produce less radiated noise than in the above example. This is because the return line currents are always the precise equal and opposite of the signal line currents. The balanced (equal and opposite) fields produced by these differential currents on the twisted-pair cable are forced to cancel each other and will not produce strong EMI.
By reducing the effect of the inductive and electric field coupling mechanisms, both electrical immunity and EMC/EMI performance are greatly enhanced.
A first aspect of the invention provides a multilayered printed circuit board for mounting a relay thereon and having a first layer with an electrically conductive plane for electrical connection to a common armature contact of the relay. The electrically conductive plane is sized to substantially cover the mounting footprint of the relay. A second layer parallel to and electrically separate from the first layer has an electrically conducting first section for electrical connection to a normally-open contact of the relay and an electrically conducting second section for electrical connection to a normally-closed contact of the relay. The first and the second sections are electrically separate from each other and in combination with each other are planar and sized to substantially cover the mounting footprint of the relay.
For mounting a plurality of relays, the first layer includes a plurality of the electrically conductive planes, each being electrically separate form each other. The second layer includes a plurality of the electrically conductive first and second sections, one pair each corresponding to each of the electrically conductive planes in the first layer, and each of the electrically conductive first and second sections are electrically separate form each other. A third layer parallel to and electrically separate from the first and second layers can be added. The third layer is electrically conductive and electrically connected to ground to form a faraday shield.
A second aspect of the invention provides a multilayered printed circuit board for mounting a relay thereon and having a first layer with a first electrically conductive plane for electrical connection to a common armature contact of the relay. The first electrically conductive plane is sized to substantially cover the mounting footprint of the relay. A second layer is parallel to and electrically separate from the first layer. The second layer has a second electrically conductive plane for electrical connection to a normally-open contact of the relay.
The second electrically conductive plane is sized to substantially cover the mounting footprint of the relay. A third layer is parallel to and electrically separate from the first and second layers and has a third electrically conductive plane for electrical connection to a normally-closed contact of the relay. The third electrically conductive plane is sized to substantially cover the mounting footprint of the relay.
For mounting a plurality of relays thereon, the first layer has a plurality of the first electrically conductive planes, each being electrically separate form each other. The second layer has a plurality of said second electrically conductive planes, each being electrically separate form each other. The third layer has a plurality of the third electrically conductive planes, each being electrically separate form each other. One each of the first, second, and third electrically conductive planes are associated with one each of the plurality of relays.
A fourth layer can be added that is parallel to and electrically separate from the first, second, and third layers. The fourth layer is electrically conductive and electrically connected to ground to form a faraday shield.
The above aspects of the invention can include relays and logic circuit components mounted on the inventive multilayered PCB. And can include relays mounted on the inventive multilayered PCB and an adjacent conventional PCB having the logic circuit components mounted thereon.
Objectives, advantages, and applications of the present invention will be made apparent by the following detailed description of embodiments of the invention.